The research proposal is designed to study the adaptation of patients with chronic obstructive lung disease who are transported to an altitude of 6300 feet for a period of three days. All patients will have spirometric evidence of chronic obstructive pulmonary disease and mild to moderate arterial hypoxemia at rest. Clinical assessment will include symptom questionnaires for fatigue, nausea, vomiting, headache and evaluation of the eye grounds, heart and lungs at altitude. Spirometry will be performed twice daily on the mountain. Full studies of resting and exercise arterial blood gases and ventilation will be performed on the third or fourth day. Limitation of exercise performance will be assessed by doing incremental exercise using three different levels of work loads both at sea level and each day on the mountain. During the exercise, saturations will be monitored by a Hewlett-Packard ear oximeter, expired air will be collected for oxygen uptake and minute ventilation during the third minute of each exercise period, and heart rate will be monitored for arrhythmias. Finally, to assess the effects of increasing altitude hypoxia on cardiac arrhythmogenic potential, Holter monitors will be carried out at sea level and at altitude for 24 hours. This study is an effort to discern whether syndromes equivalent to those seen in normal subjects at much higher altitude, such as acute high altitude sickness, retinal hemorrhages or pulmonary edema occur in patients who have an equivalent oxygen tension at moderate altitudes but who have to some extent adapted to hypoxia by the presence of disease.